1. Field of the Invention
This invention, in its preferred form, relates to apparatus for effecting a controlled sequence of precision welds in an environment non-reactive to the material of the work piece, by moving the work piece along X, Y, and Z axes with respect to the welding beam, e.g. a laser beam.
2. Description of the Prior Art
The precision laser welding apparatus of this invention relates generally to the manufacture of nuclear fuel bundle assemblies 10 as shown in FIG. 1 of the drawings. As shown, the nuclear fuel bundle assembly 10 is a self-contained unit comprised of a top nozzle assembly 12 and a bottom nozzle assemble 14, between which is disposed a matrix of nuclear fuel rods 18 arrayed in rows and columns and held in such configuration by a plurality of fuel rod grids 16. Though not shown in FIG. 1, control rods are included at selected positions within the array of nuclear fuel rods 18. The assemblies 12 and 14 and the fuel rod grids 16 provide a skeletal frame to support the fuel rods 18 and the control rods. The nuclear fuel bundle assemblies 10 are loaded into predetermined locations within a nuclear reactor and, therefore, the orientation of the fuel rods 18 with respect to each other is rigorously controlled.
The precision laser welding apparatus of this invention is, in one illustrative embodiment thereof, related to the manufacture of fuel rod grids 16 as shown in FIGS. 2A and 2E. The fuel rod grid 16 is of an approximately square configuration, whose periphery is formed by four outer grid straps 22. Each end of an outer grid strap 22 is welded by a corner seam weld 30 to the end of a perpendicularly disposed outer grid strap. A plurality of inner grid straps 20 is disposed in rows and columns perpendicular to each other, whereby a plurality of cells are formed to receive the control rods and the nuclear fuel rods 18. The inner grid straps 20 disposed along the rows and columns have complementary slots therein at each of the points 24 of intersection for receiving a perpendicularly disposed inner grid strap 20. An intersect weld 32 is formed at each of the points 24 of intersection, whereby a rigid egg crate structure is formed. Further, each of the inner grids straps 20 includes at each end a pair of tabs 26 of a size and configuration to be tightly received in either a top or bottom row of slots 28 formed in the outer grid straps 22, as shown in FIG. 2A. A slot and tab weld 34 is effected along the top and bottom rows formed by the slots 28 within the outer grid straps 22. Further, a plurality of guide sleeves 36 is disposed on the sleeve side surface of the fuel rod grid 16 to receive and guide the control rods disposed therein. A series of notch seam welds 40 securely attaches the guide sleeves 36 to corresponding notches 38 formed within the inner grid straps 20. The precision laser welding apparatus of this invention is particularly adapted to perform a series of controlled welding operations whereby each of the welds 30, 32, 34, and 40 is carried out. The precision laser welding apparatus of this invention not only controls the various parameters of generating the laser in terms of the pulse width, the pulse height of each laser pulse, and the number of pulses to be applied to each weld, but also controls the sequential positioning of the fuel rod grids 16 with respect to the laser beam. It is understood that after each such weld, the fuel rod grid 16 is repositioned and/or the focal point of the laser beam changed to effect the particular type of weld desired.
Referring now to FIGS. 2B and 2C, the plurality of resilient fingers 44 is disposed longitudinally of the inner grid straps 20 in a parallel relationship to each other. A pair of spacing fingers 46 is disposed on either side of a corresponding resilient finger 44 and serves along with the resilient finger 44 to provide a resilient grip of the nuclear fuel rods 18 that are disposed within the cell formed by the intersecting inner grid straps 20. A resilient finger 44a is disposed to the right as seen in FIG. 2C in an opposing relationship to the spacing finger 46a, whereby a nuclear fuel rod 18 is resiliently held therebetween.
The manner of assembling the inner grid straps 20 to each other as well as to the outer grid straps 22 is shown in FIG. 2D. Each of the inner grid straps 20 includes a plurality of complementary slots 52. An upper grid strap 20a has a downwardly projecting slot 52a, whereas a lower grid strap 20b has a plurality of upwardly oriented slots 52b of a configuration and size to be received within a corresponding slot 52a of the inner grid strap 20a. At each end of the inner grid strap 20, there is disposed a pair of the tabs 26 to be disposed within corresponding slots 28 of an outer grid strap 22.
As will be explained in detail later, the inner grid straps 20 are welded to each other by the intersect welds 32 as formed of projection tabs 48 and tab portions 50a and 50b. More specifically, a projection tab 48 is disposed between a corresponding set of tab portions 50a and 50b when the inner grid straps 20a and 20b are assembled together. Upon the application of a laser beam to the tab 48 and tab portions 50a and 50b, an intersect weld 32 is formed that is rigidly strong and free of contamination in accordance with the teachings of this invention. Further, each end of an outer grid strap 22 has a corner tab 54. As shown in FIG. 2D, the outer grid straps 22c and 22b have respectively corner tabs 54b and 54c that overlap each other and are seam welded together to form the corner seam weld 30.
The vanes 42 project, as seen in FIGS. 2C and 2E, from a vane side of the fuel rod grid 16 to enhance the turbulence of the water passing over the nuclear fuel rods 18. Further, as illustrated particularly in FIG. 2C, the guide sleeves 36 are aligned with cells formed by the inner grid straps 20 that are free of either a resilient finger 44 or spacing finger 46, to thereby permit the free movement of the control rod through the cell and through the guide sleeve 36.
U.S. Pat. No. 3,966,550 of Foulds et al., and U.S. Pat. No. 3,791,466 of Patterson et al., assigned to the assignee of this invention, disclose similarly configured fuel rod grids of the prior art. Each of these patents discloses a fuel rod grid wherein the inner and outer grid straps are made of a suitable metallic alloy such as Inconel, and the above identified interconnections are effected by furnace brazing. However, the zirconium alloy Zircaloy is known to have the desirable characteristic of a low neutron absorption cross section which allows for more efficient use of the nuclear fuel in the utility operation and therefore allows for a longer elapsed time between refueling by the replacement of the nuclear fuel bundle assemblies. In particular, fuel rod grids made of Zircaloy have a lower absorption rate of the neutrons generated by the fuel rods than that absorption rate of straps made with Inconel. The making of the grid straps of Zircaloy requires at least several changes in the assembly of the fuel rod grids. First, it is necessary to make the slots, whereby the inner grid straps may intersect with each other, of looser tolerances in that grid straps made of Zircaloy do not permit a force fitting thereof, i.e. to be hammered into position, but rather require controlled fit-up to allow "push-fits" of the intersecting grid straps. In addition, Zircaloy grid straps may not be brazed in that heating Zircaloy to a temperature sufficient to melt the brazing alloy would anneal the Zircaloy, resulting in a loss of mechanical strength.
The prior art has recognized the problem of fretting corrosion, wherein the surfaces of the fuel rod grids 16 and the fuel rods 18 rub against each other increasing the likelihood of weld contamination and eventual mechanical failure of the fuel rod grids 16. Fuel bundle assemblies 10 including the fuel rods 18 and grids 16 are designed to be disposed within the hostile atmosphere of a boiling water reactor (BWR) or pressurized water reactor (PWR), wherein the coolant, typically in the form of water, is super heated to temperatures in the order of 600.degree. F., i.e. the boiling point of the water coolant is raised by applying extremely high pressures thereto. Under such conditions, any contamination and, in particular, fretting corrosion is enhanced. A publication entitled "Special Features if External Corrosion of Fuel Cladding in Boiling Water Reactors", by Liv Lunde, appearing in NUCLEAR ENGINEERING AND DESIGN, (1975), describes the various mechanisms responsible for fretting corrosion. First, metallic particles are produced by grinding or by formation of welds at the points of contact between the grid 16 and its fuel rod 18. These metal particles subsequently oxidize to form an abrasive powder to increase the abrasive action. Finally, the metal beneath the protective oxide layer oxidizes due to the continuous removal of the metallic oxide by the scraping of the surfaces over each other. In particular, zirconium alloys are particularly prone to the direct oxidation of the metal by the scraping action.
It is readily contemplated that the continued contamination of the joints between the inner and outer grid straps 20 and 22 and the guide sleeves 36 of a fuel rod grid 16 will eventually lead to the joint's failure. As a result, the fuel rods 18 are subject to intense vibrations due to the high flow of the water, leading to the subsequent fuel rod rupture and to the release of the uranium oxide into the coolant water. Most of this uranium is absorbed by the ion exchangers, but small amounts may also be deposited on core components. The release of the uranium oxide into the water coolant further enhances the corrosion rate not only of the fuel grid 16 but also of the fuel rods 18. The article by Lunde particularly notes that the welding of grid and rod materials such as zirconium alloys in a contaminated welding atmosphere leads to contaminated welds and thus the problems enumerated above. In particular, there is discussed the problem of tungsten welding of Zircaloy and of the adverse effect of oxygen and water in the welding atmosphere. High amounts of oxygen will increase the hardness of the weld.
A further article, entitled "External Corrosion of Cladding in PWRs", by Stehle et al., and appearing in NUCLEAR ENGINEERING AND DESIGN, (1975), particularly describes the effect of corrosion of Zircaloy noting that at temperatures in excess of 500.degree. C. that the presence of oxygen reduces the ductility of this metal. The Stehle et al. article particularly discloses that the main problem of tungsten arc welding is the contamination by impurities in the shielding gas, including fuel particles or tungsten electrode material. In particular, such contamination appears in the form of uranium oxide that appears as a heavy white oxide layer on the fuel rods 18. In particular, the Stehle et al. article suggests that the concentrations of water and oxygen be maintained at below about 20 and 10 ppm, respectively. Though the Lunde and Stehle et al. articles do not deal with the problems of welding large Zircaloy elements and, in particular, fuel rod grids 16 made of Zircaloy, experience has shown that welds produced in a relatively impure atmosphere will produce a weld with an initially low degree of contamination that, when subjected to the harsh atmosphere of a nuclear reactor, will be particularly subject to fretting contamination. Thus, it is particularly critical that any welding of Zircaloy and, in particular, laser welding be conducted in a controlled, pure atmosphere to ensure that weld contamination is minimized and will not deteriorate under the hostile conditions of a nuclear reactor.
U.S. Pat. No. 3,555,239 of Kerth is an early example of a large body of prior art disclosing automated laser welding apparatus in which the position of the work piece, as well as the welding process, is controlled by a digital computer. Kerth shows the control of laser beams while controlling the work piece as it is moved from side to side along an X axis, horizontally forward and backward along a Y axis and vertically up and down along a Z-axis. Typically, pulse driven motors are energized by the digital computer to move the work piece rectilinearly along a selected axis. In addition, the welding is carried out within a controlled atmosphere and, in particular, the pressure and flow of gas into the welding chamber is controlled by the digital computer. Further, a counter is used to count pulses whereby the number of laser pulses applied to the work piece may likewise be controlled.
U.S. Pat. No. 4,078,167 of Banas et al. recognizes the problem of atmospheric contamination of the weld site during laser welding. Laser welding in a vacuum has been attempted, but this patent notes that vacuum welding limits the size and shape of the work piece that can be accommodated as well as increases welding time required to create the vacuum condition. Alternatively, the work piece may be totally immersed in an inert gas, or a trailer shield may provide a flow of known inert gas such as argon over the area of the work piece to be welded. In particular, U.S. Pat. No. 4,078,167 discloses a shield housing for estalishing an inert atmosphere about the weld location of the work piece as the work piece is transported beneath the shield housing. An inert gas, typically argon, is passed through a gas passing means having a plurality of openings therethrough for providing a uniform blanket of inert gas which flows over the work piece and through a passage between the shield housing and the work piece into the atmosphere. The flow of inert gas prevents to a degree atmospheric gases including oxygen and water from flowing into the welding zone. It is stated that the flow rate of an inert gas is controlled to shield the weld from reactive gases, but causes turbulence of the melted material which would produce porous and uneven welds.
U.S. Pat. No. 4,078,167 does not mention the particular metal to be welded and does not contemplate the laser welding of Zircaloy as for the above-described fuel rod grid. Zircaloy is known to be highly reactive to oxygen, nitrogen, and water as found in the atmosphere, and welding tests leading to this invention have demonstrated conclusively that inert gas flow around the immediate weld area does not provide adequate shielding for the laser welding of Zircaloy. In accordance with the teachings of this invention, an atmosphere of an inert gas such as argon has been established with a purity in the order of 10 PPM, which degree of purity is not contemplated by U.S. Pat. No. 4,078,167.
The above discussion of the prior art illustrates the significant problems in achieving automated laser welding of a highly reactive material such as Zircaloy, wherein the work piece is sequentially moved under an automated controller to effect a number of precision welds. As enumerated above, it is necessary to move the work piece, e.g. the laser welded grid 16 as described above, along each of its X, Y, and Z axes with respect to the focused laser beam while maintaining the surrounding atmosphere at an exceptionally high degree of purity to avoid contamination of the welded material. In particular, the laser welding systems of the prior art have not been particularly adapted to automated control, wherein it is desired to not only move the work piece in three dimensions but also to maintain the work piece in a pure environment that is non-reactive to the material of which the work piece is made. It is evident that if such laser welding apparatus could be automated, that its rate of production could be also increased. In addition, the prior art has not achieved the desired efficiency in terms of loading and unloading the apparatus in fascile manner, while at the same time adapting such apparatus to be easily cleaned and maintained. In this regard, any welding chamber must be periodically cleaned to remove debris that is produced during the welding process. In addition, it is desired to achieve a high degree of laser efficiency, even while the work piece is being moved through a sequence of positions in three dimensions with respect to the laser beam. In addition, there are problems of effecting precise welds in parts of small dimensions and, in particular, of maintaining the power level of the impinging laser beam at precise levels for different types of welds, noting the attenuation of laser output as a laser system including the laser rod and excitation lamps is used at high work duty ratios over an extended period of time, and the effects of laser welding debris.